A variety of metal and polymeric composites are known and used in many, varied applications. One important use for a metal/polymer composite is as a shield for electromagnetic and radio frequency waves. The interference caused by such waves in electronic devices is commonly referred to as electromagnetic interference (EMI) or radio frequency interference (RFI) (hereinafter jointly referred to as EMI). EMI shielding is often placed around an EMI source to prevent it from radiating EMI and interfering with surrounding devices. Also, the devices themselves may be provided with EMI shielding in an effort to shield the device from incoming electromagnetic (EM) radiation.
Another important use for a metal/polymer composite is for the protection of sensitive electronic parts from static charges. Static charge build-up can result from, for example, friction between surfaces, and can lead to a build-up of a high electrical potential. A sensitive electronic part that may come into proximity or contact with a statically charged surface can be destroyed or damaged. During shipping or handling, shielding of an electronic part from static electricity can be accomplished by placing the part in an electrically conductive metal or metal/polymer container, with the metal providing a preemptive surface to drain away any static charge.
Many applications require that the shielding be thermoformed into a particular shape or structure. A thermoforming process comprises heating the material and forming it into a desired shape. For example, thermoforming is used to make various types of containers or housings. Thermoforming is usually accomplished by heating a thermoplastic sheet above its softening point, and forcing it against a mold by applying vacuum, air, or mechanical pressure. On cooling, the contour of the mold is reproduced in detail.
Metal is a well-known effective shield against EMI or static charges. Metal/polymer composites having continuous coatings of metal are known. Continuous metal coatings are typically deposited by vapor deposition, sputtering, plating or the like. To obtain coatings sufficiently thick to provide good electrical conductivity, these methods are time consuming and relatively expensive. Metal is also relatively heavy compared to polymeric materials, and therefore, where weight is a factor the amount of metal is desirably reduced.
As an alternative to continuous metal coatings, high loadings of short, straight, staple metal fibers have been used in EMI shielding. In such EMI shielding the effectiveness of the shield is related to the dimensions of the spaces between the metal fibers. Contrary to what might commonly be expected, the amount of EMI that passes through a given void is dependent on the length of the void's longest dimension, and not on the total area of the void. For illustration, a 1.0 mm by 1.0 mm void (area--1.0 mm.sup.2) is believed to let less EMI pass through than would a 3.0 mm by 0.05 mm void (area 0.15 mm.sup.2) even though the 1.0 mm.sup.2 space has greater than six times the area of the 0.15 mm.sup.2 space. This is sometimes referred to as the "slot effect."
Accordingly, even extremely thin openings, such as those between mating parts, must be avoided where the void or opening has a substantial longitudinal dimension. To accomplish this end, it is desirable that the surfaces of EMI shielded enclosures include conductive coatings. An enclosure can be effectively sealed without leaving thin cracks between adjacent enclosure parts (e.g. a box and a box lid), by providing conductive surfaces on adjacent surfaces of the shielded enclosure. For this reason fibers, particles, or flakes of metal extruded or laminated into bulk polymer are not optimally effective unless the metal is distributed throughout the polymer to specifically include the sealing surfaces.
The effectiveness of EMI shielding is also directly related to the overall electrical conductivity of the polymer or the polymer coating, i.e., higher electrical conductivity gives better EMI shielding. When electrical conductivity is obtained by mixing conductive fibers into a polymer or by forming a coating of such fibers, the overall conductivity is dependent on the conductivity of the individual fibers and the amount of contact between the fibers. Short staple fibers require higher bulk loading, or for coatings, higher surface concentrations to obtain sufficient contact between the fibers. The number of contacts is significantly reduced during the thermoforming process, reducing the effectiveness of the EMI shielding.
Electrical conductivity can be enhanced by forming pressure welds or sintered bonds between the fibers, but the overall flexibility and ductility of a welded fiber mat is then reduced. A sintered or otherwise bonded metal-fiber/polymer composite sheet loses its ability to be thermoformed except at thermoforming stresses sufficient to break the bonds between the fibers, allowing the fibers to slip past one another. However, when the bonds between fibers are broken, the electrical conductivity and EMI or static shielding properties of the composite are drastically reduced.
Electrical conductivity could be enhanced by using longer fibers that would require fewer fiber-fiber contacts to maintain continuous electrical conductivity. However, metal fibers in their solid state stretch very little, if at all, under conditions used for thermoforming polymers Therefore, the fibers must slip relative to the polymer to keep from breaking during thermoforming. Curly fibers in their solid state may accomodate some of the strain by straightening, but there is still a slip component of the fiber relative to the polymer matrix as the fiber changes its shape during deformation of the polymer matrix. The net effect of this slip is a dramatic increase in the thermoforming stress needed for plastic flow of the composite. In addition, at concave surfaces or corners the fibers may pop out of the surface.
Accordingly, there is a need for a thin metal/polymer composite for use as a conductive coating, the composite including metal fibers to give efficient electrical conductivity for the conductive coating. For EMI shielding the coating will have a uniform coverage of metal fibers in all planar directions so as to not have voids, especially long narrow voids, that allow EM radiation to leak through the coating. There is also a need for a conductive coating that includes a mat of fine metal fibers having sufficient voids to be transparent, the coating providing static shielding. There is also a need for such a metal/polymer composite to be thermoformable without loss of electrical continuity, or EMI or static shielding properties.
There is a further need for an economical and simple method of making a non-woven metal mat of randomly-arranged, fine metal strands that can be sintered and/or embedded into a polymeric substrate for use as an EMI shield. It may also be desirable for the metal strands to be curly, forming an entangled self-supporting web.